There exist various techniques for improving the contrast of a liquid crystal display. The following is examples disclosed in patent documents 1 to 7.
Patent document 1 discloses a technique of optimizing the relative amount and surface area ratio of the yellow component of pigment in a color filter to improve the contrast ratio. The technique successfully addresses the problem of poor contrast ratio of a liquid crystal display caused by pigment molecules in the color filter scattering and depolarizing polarized light. Patent document 1 states that the contrast ratio of a liquid crystal display improves from 280 to 420.
Patent document 2 discloses a technique of increasing the transmittance and polarizing capability of a polarizer to improve the contrast ratio. Patent document 2 states that the contrast ratio of a liquid crystal display improves from 200 to 250.
Patent documents 3 and 4 disclose a technique for improving contrast in guest-host mode which exploits absorption of light by a dichroic pigment.
Patent document 3 describes a method of improving contrast by way of a structure in which two guest-host liquid crystal cells are provided with a quarter-wave plate interposed between the two cells. Patent document 3 discloses omission of polarizers.
Patent document 4 discloses a liquid crystal display element in which a dichroic pigment is mixed with a liquid crystal used in dispersive liquid crystal mode. Patent document 4 states a contrast ratio of 101.
The techniques disclosed in patent documents 3 and 4 show relatively low contrast when compared to the other schemes. To further improve the contrast, various methods may be available: the light absorption by the dichroic pigment may be improved, the pigment content increased, or the thickness of the guest-host liquid crystal cell(s) increased. All these methods however lead to new problems, such as technical problems, poor reliability, and poor response properties.
Patent documents 5 and 6 disclose a method of improving contrast by an optical compensation scheme. The documents describe a liquid crystal panel and a liquid crystal display panel provided between a pair of polarizers. The liquid crystal panel performs optical compensation.
Patent document 5 improves a retardation contrast ratio from 14 to 35 in STN mode using a display cell and a liquid crystal cell which is provided to perform optical compensation.
Patent document 6 improves a contrast ratio from 8 to 100 by disposing a liquid crystal cell for optical compensation. The cell compensates for wavelength dependence of a liquid crystal display cell in, for example, TN mode when the display cell is displaying black.
Although the techniques disclosed in the patent documents achieve a 1.2- to 10-fold or even greater increase in contrast ratio, the absolute value of contrast ratio is no higher than about 35 to 420.
Another contrast enhancing technique is disclosed in patent document 7, for example. The document teaches a complex liquid crystal display in which two liquid crystal panels are stacked in such a manner that polarizers form crossed Nicols. Patent document 7 states that the stacking of two panels increases the contrast ratio to three to four digit values whilst the panel, if used alone, shows a contrast ratio of 100.    Patent document 1: Japanese Unexamined Patent Publication (Tokukai) 2001-188120 (published Jul. 10, 2001)    Patent document 2: Japanese Unexamined Patent Publication (Tokukai) 2002-90536 (published Mar. 27, 2002)    Patent document 3: Japanese Unexamined Patent Publication 63-25629/1988 (Tokukaisho 63-25629; published Feb. 3, 1988)    Patent document 4: Japanese Unexamined Patent Publication 5-2194/1993 (Tokukaihei 5-2194; published Jan. 8, 1993)    Patent document 5: Japanese Unexamined Patent Publication 64-49021/1989 (Tokukaisho 64-49021; published Feb. 23, 1989)    Patent document 6: Japanese Unexamined Patent Publication 2-23/1990 (Tokukaihei 2-23; published Jan. 5, 1990)    Patent document 7: Japanese Unexamined Patent Publication 5-88197/1993 (Tokukaihei 5-88197; published Apr. 9, 1993)